1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser beam printer, a digital copier or such, and in particular, to an optical scanning apparatus for carrying out optical writing and scanning of a photosensitive body or such, and an image forming apparatus employing it.
2. Description of the Related Art
Recently, a color laser printer, a color copier and so forth have been rapidly put into practical use. For this purpose, a configuration by which a plurality of scan lines can be produced on a plurality of photosensitive bodies is demanded in an optical scanning apparatus used there. As a system of meeting the demand, various manners can be considered. For example, a tandem type can be applied in which four photosensitive bodies corresponding to respective color components, i.e., C, M, Y and K (i.e., cyan, magenta, yellow and black, respectively) are provided, for example.
In an optical scanning apparatus in the tandem type as shown in FIG. 11 and FIG. 12 (a), in order to obtain a necessary spacing Z to separate light beams for the respective photosensitive bodies, a polygon mirror of two stages is applied. In this case, if a polygon mirror of a single stage is applied instead, the polygon mirror should have a large thickness in a sub-scanning direction accordingly, which may result in difficulty in an increase in a processing speed or a cost reduction. Patent Document 1 discloses an optical scanning apparatus employing a light source unit suitable for such a tandem opposed scanning type. In this optical scanning apparatus, a plurality of light emitting devices and so forth are provided in a plurality of stages in a sub-scanning direction on a plane opposite to an optical deflector.
Further, a so-called oblique entrance optical system is known as a scanning optical system for a reduced cost suitable to the tandem type configuration in which light beams are applied to a deflection reflective surface of an optical deflector at angles different in a sub-scanning direction with respect to a normal of the deflection reflective surface. As can be seen from FIG. 12 (b), in this system, a thickness of the polygon mirror should not necessarily be increased for the purpose of ensuring the necessary spacing Z for separating the light beams for the respective to-be-scanned surfaces. Thereby, the oblique entrance optical system may be reduced in the cost.
However, the oblique entrance optical system may involve a problem of scan line bending. An actual amount of the scan line bending depends on an oblique entrance angle in a sub-scanning direction of each light beam, and color drift may occur when latent images drawn with these light beams are visualized by toners of respective colors and are superposed together. Further, as a result of the oblique entrance, the light beam may be applied to the scanning lens in a twisted manner, wavefront aberration may increase, especially optical performance may remarkably degrade at a peripheral image height, and a beam spot diameter may increase. As a result, high quality image formation may become difficult. In order to solve the problem, the oblique entrance angle should preferably be reduced in the optical system.
In the oblique entrance optical system, light beam entrance is made from the light source for a rotational axis of the polygon mirror. Accordingly, the oblique entrance angle increases in order to avoid interference with the scanning lens when the light source is disposed at a position just corresponding to the scanning lens in the main scanning direction. In order to reduce the oblique entrance angle, various methods may be applied. However, an increase in the light path length of the front side optical system for this purpose may result in an increase in the apparatus size, which may result in difficulty in meeting the needs of the market.
When the above-mentioned oblique entrance optical system is applied to the tandem type configuration, an opposed type scanning oblique entrance optical system or a single side scanning oblique entrance optical system may be applied. The first one is such that, as shown in FIG. 13 (a), two light beams are deflected by each of opposite surfaces of the polygon mirror. As shown, since only two light beams can be applied to each side, light beam entrance is made symmetrically with respect to an optical standard plane at the same oblique entrance angle. In contrast thereto, the second one is such that, as shown in FIG. 13 (b), all the light beams are applied to the same surface of the polygon mirror. In this case, as shown, the light source part can be provided concentrically only on the single side, and also, the lens closest to the polygon mirror (optical deflector) can be shared for all the four light beams. Thus, the number of required components can be reduced in the single side scanning oblique entrance optical system. Patent Document 2 discloses an optical scanning apparatus employing a light source unit suitable to the opposed type scanning oblique entrance optical system. In this optical scanning apparatus, light beams emitted are inclined in their optical axes in such a manner that these light beams intersect each other at a predetermined angle θ, and required light sources are integrated into a single unit.
It is noted that Patent Document 1 denotes Japanese Laid-open Patent Application No. 2002-90672; and Patent Document 2 denotes Japanese Laid-open Patent Application No. 2004-271906.
The configuration of Patent Document 1 may not be advantageous in that the polygon mirror in the two stages may obstruct an increase in the processing speed or the cost reduction. Also, the required number of components may increase since the light source parts and imaging optical systems should be provided on both sides of the polygon mirror, which may obstruct the cost reduction.
The configuration of the Patent Document 2 also may have a problem the same as that of Patent Document 1 that the light source parts and imaging optical systems provided on both sides of the polygon mirror may obstruct the cost reduction. Further, when this configuration is applied to the single side scanning oblique entrance optical system, a plurality of the oblique entrance angles with respect to the optical standard plane are required, and thus, the required number of optical units may increase. This is because light beams from a plurality of light sources included in a single light source unit are applied to the polygon mirror at the equal oblique entrance angles with respect to the optical standard plane, respectively. Therefore, when the configuration is applied to the single side scanning oblique entrance optical system in which four light beams should be applied from one side, a plurality of oblique entrance angles are required even when the optically plane symmetrical configuration is applied.
As a result, the required number of the different light source units each having ‘the oblique entrance angle symmetrical with respect to the optical standard plane’ should be equal to the number of the required oblique entrance angles. Thus, the required number of units increases, and thus, the cost reduction of the optical system may not be achieved.
The above-mentioned two configurations of Patent Documents 1 and 2 assume the opposed type scanning oblique entrance optical system, and thus, may involve the problem of the increase in the required number of components. In contrast thereto, the single side scanning oblique entrance optical system may be advantageous in terms of reduction in the required number of components. However, in the single side scanning oblique entrance optical system (see FIG. 13 (b)), the four light beams should be applied to the common surface of the polygon mirror. In other words, although these four beams may be applied at respective angles having no relationship thereamong, it is necessary that two sets of light beams, each set including two light beams which form a pair with respect to the optical standard plane, are provided, and these sets of light beams are applied at oblique entrance angles different from each other.